Tens of thousands of Russian opposition supporters thronged the streets of Moscow on Saturday to keep up the pressure on President Vladimir Putin four months into his new mandate.

A sea of protestors, brandishing the red flags of socialism, nationalist tricolours or liberal slogans, filled the avenue in Moscow named after the great Soviet physicist and celebrated dissident Andrei Sakharov.

Some 40,000 people turned out for the "March of Millions", according to an AFP estimate, in line with organisers' expectations but down on the first protests that rocked Moscow in December, which drew more than 100,000.

Even some of the most ardent and popular opposition leaders admitted that their movement, while certainly not dying after Putin's return for a third term, still lacked the energy to make an immediate impact.

Alexei Navalny, a blogger who has hinted that he might run against Putin in the future, told the crowd to prepare for a long haul and turn up to rallies as if going to work.

"All that we are asking for is something simply called freedom, nothing more than equality, simply human dignity," the 36-year-old said.

"No one will give us freedom except for ourselves. Hope and perseverance will bring us victory," he said to cheers from the crowd.

Equally worrying to Navalny and the new generation of politically active Russians, few people in the regions turned out to demonstrate against Putin, the country's most popular and powerful figure of the past decade.

Just 2,500 people turned out for an opposition protest in the second city of Saint Petersburg, once the hotbed of political activism, an AFP correspondent said. Another 800 people rallied in the Urals city of Yekaterinburg.

And a protest in the Far East city of Vladivostok only mustered a few dozen people, police and organisers said.

"People are afraid to join (protests) in the provinces," said prominent political analyst Vladimir Pribylovsky.

"There, if they detain you at a meeting, you risk losing your job or getting kicked out of school."

Police and organisers gave vastly different estimates for Moscow itself. Police put the figure at 14,000 people, while far-left leader Sergei Udaltsov estimated at least 150,000.

Split between liberals, nationalists and the extreme left, the anti-Putin opposition has been struggling with its own divisions and accusations it lacks any coherent message beyond hostility to the Kremlin.

The Kremlin stressed that Putin himself had more important things to occupy his time.

"Vladimir Vladimirovich (Putin) had a full working day," said his official spokesman.

"Unfortunately, we did not have the opportunity to monitor this event," he said.

The protest was given extra impetus by the expulsion from parliament of anti-Putin deputy Gennady Gudkov over an alleged conflict of interests that could now lead to his prosecution.

His supporters say the move was in reprisal for his opposition to Putin.

To some of the afternoon's biggest cheers, Gudkov warned the authorities that if there was no reform "they will either end up standing in blood, or they will be overthrown".

It is also the first mass action since the two-year jail terms handed down last month to three members of Pussy Riot for an anti-Putin protest in an Orthodox cathedral.

Their plight has become a rallying cause for many in the opposition. Protestor Anna Roskina, 33, a psychologist, held a banner saying "Freedom for Pussy Riot! Freedom for us all!"

There were no reports of violence or arrests throughout nine-hour gathering, in contrast to the May 6 march on the eve of Putin's inauguration. That ended with helmeted police wielding clubs against protesters and hundreds of arrests.

But Udaltsov was a one of a handful of people detained by police after they stayed past the 10:00 pm (1800 GMT) deadline for the event to finish.

They are expected to be quickly released, in line with a string of arrests and subsequent quick releases at similar functions.

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